1. Field of the Invention
This invention relates to dielectric ceramics for high frequencies, which indicates a particularly high dielectric constant and a high Q value at micro-wave frequencies.
2. Description of the Prior Art
The dielectric ceramics for high frequency have a wide range of applications in the fields of portable wireless apparatuses, satellite broadcasting, micro-wave integrated circuits, etc. Above all, these dielectric ceramics are used as the dielectric resonators for improving the filter characteristics and frequency stability. The following characteristics of the dielectric resonator are required when it is used in the microwave region: since the size of the dielectric resonator is inversely proportional to the square root of the dielectric constant, it should have .circle. a high dielectric constant for the reduction in size of the resonator; .circle. a low dielectric loss; and .circle. an excellent temperature stability against resonant frequency.
As well known dielectric ceramics having such characteristics as mentioned above, there can be exemplified a compound as disclosed in, for example, Japanese Unexamined Patent Publication No. 198505/1986 to be represented by the following general formula: aSrO bCaO.multidot.cTiO.sub.2 .multidot.dSnO.sub.2 (where 0.39.ltoreq.a.ltoreq.0.70, 0.ltoreq.b.ltoreq.0.28, 0.27.ltoreq.c.ltoreq.0.35, 0.ltoreq.d.ltoreq.0.04, a+b+c+d=1), so forth.
However, in the dielectric ceramic materials for high frequencies, it is generally very difficult to attain both a high relative dielectric constant and a low dielectric loss. To be precise, the relative dielectric constant of the above-mentioned conventional dielectric ceramics is 40 or so, and Q value (1/dielectric loss) thereof indicates a range from several thousands to 15,000 or so, when measured at 9 GHz. While such dielectric ceramics exist in a comparatively large quantity, the ceramics materials having relative high dielectric constant are recognized to have a tendency to increase their dielectric loss, remarkably, and the current situation is such that those ceramics materials having their relative dielectric constant of 80 and indicating the Q value of 1,000 at the same measuring frequency, as mentioned above are available only in a very small quantity.